1======================================== 2A description of what robust futexes are 3======================================== 4 5:Started by: Ingo Molnar <mingo@redhat.com> 6 7Background 8---------- 9 10what are robust futexes? To answer that, we first need to understand 11what futexes are: normal futexes are special types of locks that in the 12noncontended case can be acquired/released from userspace without having 13to enter the kernel. 14 15A futex is in essence a user-space address, e.g. a 32-bit lock variable 16field. If userspace notices contention (the lock is already owned and 17someone else wants to grab it too) then the lock is marked with a value 18that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT) 19syscall is used to wait for the other guy to release it. The kernel 20creates a 'futex queue' internally, so that it can later on match up the 21waiter with the waker - without them having to know about each other. 22When the owner thread releases the futex, it notices (via the variable 23value) that there were waiter(s) pending, and does the 24sys_futex(FUTEX_WAKE) syscall to wake them up. Once all waiters have 25taken and released the lock, the futex is again back to 'uncontended' 26state, and there's no in-kernel state associated with it. The kernel 27completely forgets that there ever was a futex at that address. This 28method makes futexes very lightweight and scalable. 29 30"Robustness" is about dealing with crashes while holding a lock: if a 31process exits prematurely while holding a pthread_mutex_t lock that is 32also shared with some other process (e.g. yum segfaults while holding a 33pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need 34to be notified that the last owner of the lock exited in some irregular 35way. 36 37To solve such types of problems, "robust mutex" userspace APIs were 38created: pthread_mutex_lock() returns an error value if the owner exits 39prematurely - and the new owner can decide whether the data protected by 40the lock can be recovered safely. 41 42There is a big conceptual problem with futex based mutexes though: it is 43the kernel that destroys the owner task (e.g. due to a SEGFAULT), but 44the kernel cannot help with the cleanup: if there is no 'futex queue' 45(and in most cases there is none, futexes being fast lightweight locks) 46then the kernel has no information to clean up after the held lock! 47Userspace has no chance to clean up after the lock either - userspace is 48the one that crashes, so it has no opportunity to clean up. Catch-22. 49 50In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot 51is needed to release that futex based lock. This is one of the leading 52bugreports against yum. 53 54To solve this problem, the traditional approach was to extend the vma 55(virtual memory area descriptor) concept to have a notion of 'pending 56robust futexes attached to this area'. This approach requires 3 new 57syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and 58FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether 59they have a robust_head set. This approach has two fundamental problems 60left: 61 62 - it has quite complex locking and race scenarios. The vma-based 63 approach had been pending for years, but they are still not completely 64 reliable. 65 66 - they have to scan _every_ vma at sys_exit() time, per thread! 67 68The second disadvantage is a real killer: pthread_exit() takes around 1 69microsecond on Linux, but with thousands (or tens of thousands) of vmas 70every pthread_exit() takes a millisecond or more, also totally 71destroying the CPU's L1 and L2 caches! 72 73This is very much noticeable even for normal process sys_exit_group() 74calls: the kernel has to do the vma scanning unconditionally! (this is 75because the kernel has no knowledge about how many robust futexes there 76are to be cleaned up, because a robust futex might have been registered 77in another task, and the futex variable might have been simply mmap()-ed 78into this process's address space). 79 80This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that 81normal kernels can turn it off, but worse than that: the overhead makes 82robust futexes impractical for any type of generic Linux distribution. 83 84So something had to be done. 85 86New approach to robust futexes 87------------------------------ 88 89At the heart of this new approach there is a per-thread private list of 90robust locks that userspace is holding (maintained by glibc) - which 91userspace list is registered with the kernel via a new syscall [this 92registration happens at most once per thread lifetime]. At do_exit() 93time, the kernel checks this user-space list: are there any robust futex 94locks to be cleaned up? 95 96In the common case, at do_exit() time, there is no list registered, so 97the cost of robust futexes is just a simple current->robust_list != NULL 98comparison. If the thread has registered a list, then normally the list 99is empty. If the thread/process crashed or terminated in some incorrect 100way then the list might be non-empty: in this case the kernel carefully 101walks the list [not trusting it], and marks all locks that are owned by 102this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if 103any). 104 105The list is guaranteed to be private and per-thread at do_exit() time, 106so it can be accessed by the kernel in a lockless way. 107 108There is one race possible though: since adding to and removing from the 109list is done after the futex is acquired by glibc, there is a few 110instructions window for the thread (or process) to die there, leaving 111the futex hung. To protect against this possibility, userspace (glibc) 112also maintains a simple per-thread 'list_op_pending' field, to allow the 113kernel to clean up if the thread dies after acquiring the lock, but just 114before it could have added itself to the list. Glibc sets this 115list_op_pending field before it tries to acquire the futex, and clears 116it after the list-add (or list-remove) has finished. 117 118That's all that is needed - all the rest of robust-futex cleanup is done 119in userspace [just like with the previous patches]. 120 121Ulrich Drepper has implemented the necessary glibc support for this new 122mechanism, which fully enables robust mutexes. 123 124Key differences of this userspace-list based approach, compared to the 125vma based method: 126 127 - it's much, much faster: at thread exit time, there's no need to loop 128 over every vma (!), which the VM-based method has to do. Only a very 129 simple 'is the list empty' op is done. 130 131 - no VM changes are needed - 'struct address_space' is left alone. 132 133 - no registration of individual locks is needed: robust mutexes don't 134 need any extra per-lock syscalls. Robust mutexes thus become a very 135 lightweight primitive - so they don't force the application designer 136 to do a hard choice between performance and robustness - robust 137 mutexes are just as fast. 138 139 - no per-lock kernel allocation happens. 140 141 - no resource limits are needed. 142 143 - no kernel-space recovery call (FUTEX_RECOVER) is needed. 144 145 - the implementation and the locking is "obvious", and there are no 146 interactions with the VM. 147 148Performance 149----------- 150 151I have benchmarked the time needed for the kernel to process a list of 1 152million (!) held locks, using the new method [on a 2GHz CPU]: 153 154 - with FUTEX_WAIT set [contended mutex]: 130 msecs 155 - without FUTEX_WAIT set [uncontended mutex]: 30 msecs 156 157I have also measured an approach where glibc does the lock notification 158[which it currently does for !pshared robust mutexes], and that took 256 159msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls 160userspace had to do. 161 162(1 million held locks are unheard of - we expect at most a handful of 163locks to be held at a time. Nevertheless it's nice to know that this 164approach scales nicely.) 165 166Implementation details 167---------------------- 168 169The patch adds two new syscalls: one to register the userspace list, and 170one to query the registered list pointer:: 171 172 asmlinkage long 173 sys_set_robust_list(struct robust_list_head __user *head, 174 size_t len); 175 176 asmlinkage long 177 sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, 178 size_t __user *len_ptr); 179 180List registration is very fast: the pointer is simply stored in 181current->robust_list. [Note that in the future, if robust futexes become 182widespread, we could extend sys_clone() to register a robust-list head 183for new threads, without the need of another syscall.] 184 185So there is virtually zero overhead for tasks not using robust futexes, 186and even for robust futex users, there is only one extra syscall per 187thread lifetime, and the cleanup operation, if it happens, is fast and 188straightforward. The kernel doesn't have any internal distinction between 189robust and normal futexes. 190 191If a futex is found to be held at exit time, the kernel sets the 192following bit of the futex word:: 193 194 #define FUTEX_OWNER_DIED 0x40000000 195 196and wakes up the next futex waiter (if any). User-space does the rest of 197the cleanup. 198 199Otherwise, robust futexes are acquired by glibc by putting the TID into 200the futex field atomically. Waiters set the FUTEX_WAITERS bit:: 201 202 #define FUTEX_WAITERS 0x80000000 203 204and the remaining bits are for the TID. 205 206Testing, architecture support 207----------------------------- 208 209I've tested the new syscalls on x86 and x86_64, and have made sure the 210parsing of the userspace list is robust [ ;-) ] even if the list is 211deliberately corrupted. 212 213i386 and x86_64 syscalls are wired up at the moment, and Ulrich has 214tested the new glibc code (on x86_64 and i386), and it works for his 215robust-mutex testcases. 216 217All other architectures should build just fine too - but they won't have 218the new syscalls yet. 219 220Architectures need to implement the new futex_atomic_cmpxchg_inatomic() 221inline function before writing up the syscalls. 222