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Message-Id: <498d773729dc31d4e9cb0e58308254f2c4332d20.1514985618.git.Jens.Gustedt@inria.fr> Date: Wed, 3 Jan 2018 14:17:12 +0100 From: Jens Gustedt <Jens.Gustedt@...ia.fr> To: musl@...ts.openwall.com Subject: [PATCH 6/7] implement __unlock_requeue We need a special case of unlock when we might take advantage of the futex requeue feature to move a thread from a condition queue to the one of the correponding mutex --- src/internal/__lock.h | 8 ++++++++ src/internal/libc.h | 2 ++ src/thread/__lock.c | 17 +++++++++++++++++ 3 files changed, 27 insertions(+) diff --git a/src/internal/__lock.h b/src/internal/__lock.h index f16d6176..0c2e9bfb 100644 --- a/src/internal/__lock.h +++ b/src/internal/__lock.h @@ -19,3 +19,11 @@ static inline void __unlock_fast(volatile int *l) } } } + +static inline void __unlock_requeue_fast(volatile int *l, volatile int *r, int shared) +{ + extern void __unlock_requeue_slow(volatile int*, volatile int*, int); + /* We have to check if l[0] had been touched at all. */ + if (l[0] < 0 && (a_fetch_add(l, -(INT_MIN + 1)) != (INT_MIN + 1))) + __unlock_requeue_slow(l, r, shared); +} diff --git a/src/internal/libc.h b/src/internal/libc.h index a594d0c5..999b4cba 100644 --- a/src/internal/libc.h +++ b/src/internal/libc.h @@ -50,6 +50,8 @@ extern char *__progname, *__progname_full; void __lock_slow(volatile int *, int) ATTR_LIBC_VISIBILITY; void __lock(volatile int *) ATTR_LIBC_VISIBILITY; void __unlock(volatile int *) ATTR_LIBC_VISIBILITY; +void __unlock_requeue(volatile int *, volatile int *, int) ATTR_LIBC_VISIBILITY; +void __unlock_requeue_slow(volatile int *, volatile int *, int) ATTR_LIBC_VISIBILITY; int __lockfile(FILE *) ATTR_LIBC_VISIBILITY; void __unlockfile(FILE *) ATTR_LIBC_VISIBILITY; #define LOCK(x) __lock(x) diff --git a/src/thread/__lock.c b/src/thread/__lock.c index a3d8d4d0..b0b0c67c 100644 --- a/src/thread/__lock.c +++ b/src/thread/__lock.c @@ -19,6 +19,23 @@ weak_alias(__lock_fast, __lock); weak_alias(__unlock_fast, __unlock); +weak_alias(__unlock_requeue_fast, __unlock_requeue); + +/* __unlock_requeue handles the case where we have a second futex + * queue to which the a waiting thread should be scheduled if shared + * is false. This is e.g the case when l is the futex of a condition + * variable and r is the one of a private mutex: if the mutex is + * private, it is more efficient to just insert the thread in the + * queue of that mutex, instead of waking him up and put it back to + * sleep immediately when trying to lock the mutex. */ + +void __unlock_requeue_slow(volatile int *l, volatile int *r, int shared) +{ + if (shared) __wake(l, 1, 1); + else __syscall(SYS_futex, l, FUTEX_REQUEUE|FUTEX_PRIVATE, 0, 1, r) != -ENOSYS + || __syscall(SYS_futex, l, FUTEX_REQUEUE, 0, 1, r); +} + void __lock_slow(volatile int *l, int current) { -- 2.15.1
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