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nptl: Remove g_refs from condition variables

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This variable used to be needed to wait in group switching until all sleepers
have confirmed that they have woken. This is no longer needed. Nothing waits
on this variable so there is no need to track how many threads are currently
asleep in each group.

Signed-off-by: Malte Skarupke <malteskarupke@fastmail.fm>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
This commit is contained in:
Malte Skarupke 2024-12-04 07:56:38 -05:00 committed by Carlos O'Donell
parent 4f7b051f8e
commit c36fc50781
4 changed files with 9 additions and 60 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

52
nptl/pthread_cond_wait.c
View File

@ -143,23 +143,6 @@ __condvar_cancel_waiting (pthread_cond_t *cond, uint64_t seq, unsigned int g,
}
}
/* Wake up any signalers that might be waiting. */
static void
__condvar_dec_grefs (pthread_cond_t *cond, unsigned int g, int private)
{
/* Release MO to synchronize-with the acquire load in
__condvar_quiesce_and_switch_g1. */
if (atomic_fetch_add_release (cond->__data.__g_refs + g, -2) == 3)
{
/* Clear the wake-up request flag before waking up. We do not need more
than relaxed MO and it doesn't matter if we apply this for an aliased
group because we wake all futex waiters right after clearing the
flag. */
atomic_fetch_and_relaxed (cond->__data.__g_refs + g, ~(unsigned int) 1);
futex_wake (cond->__data.__g_refs + g, INT_MAX, private);
}
}
/* Clean-up for cancellation of waiters waiting for normal signals. We cancel
our registration as a waiter, confirm we have woken up, and re-acquire the
mutex. */
@ -171,8 +154,6 @@ __condvar_cleanup_waiting (void *arg)
pthread_cond_t *cond = cbuffer->cond;
unsigned g = cbuffer->wseq & 1;
__condvar_dec_grefs (cond, g, cbuffer->private);
__condvar_cancel_waiting (cond, cbuffer->wseq >> 1, g, cbuffer->private);
/* FIXME With the current cancellation implementation, it is possible that
a thread is cancelled after it has returned from a syscall. This could
@ -327,15 +308,6 @@ __condvar_cleanup_waiting (void *arg)
sufficient because if a waiter can see a sufficiently large value, it could
have also consume a signal in the waiters group.
It is essential that the last field in pthread_cond_t is __g_signals[1]:
The previous condvar used a pointer-sized field in pthread_cond_t, so a
PTHREAD_COND_INITIALIZER from that condvar implementation might only
initialize 4 bytes to zero instead of the 8 bytes we need (i.e., 44 bytes
in total instead of the 48 we need). __g_signals[1] is not accessed before
the first group switch (G2 starts at index 0), which will set its value to
zero after a harmless fetch-or whose return value is ignored. This
effectively completes initialization.
Limitations:
* This condvar isn't designed to allow for more than
@ -440,21 +412,6 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
if ((int)(signals - lowseq) >= 2)
break;
/* No signals available after spinning, so prepare to block.
We first acquire a group reference and use acquire MO for that so
that we synchronize with the dummy read-modify-write in
__condvar_quiesce_and_switch_g1 if we read from that. In turn,
in this case this will make us see the advancement of __g_signals
to the upcoming new g1_start that occurs with a concurrent
attempt to reuse the group's slot.
We use acquire MO for the __g_signals check to make the
__g1_start check work (see spinning above).
Note that the group reference acquisition will not mask the
release MO when decrementing the reference count because we use
an atomic read-modify-write operation and thus extend the release
sequence. */
atomic_fetch_add_acquire (cond->__data.__g_refs + g, 2);
// Now block.
struct _pthread_cleanup_buffer buffer;
struct _condvar_cleanup_buffer cbuffer;
@ -471,18 +428,11 @@ __pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
if (__glibc_unlikely (err == ETIMEDOUT || err == EOVERFLOW))
{
__condvar_dec_grefs (cond, g, private);
/* If we timed out, we effectively cancel waiting. Note that
we have decremented __g_refs before cancellation, so that a
deadlock between waiting for quiescence of our group in
__condvar_quiesce_and_switch_g1 and us trying to acquire
the lock during cancellation is not possible. */
/* If we timed out, we effectively cancel waiting. */
__condvar_cancel_waiting (cond, seq, g, private);
result = err;
goto done;
}
else
__condvar_dec_grefs (cond, g, private);
/* Reload signals. See above for MO. */
signals = atomic_load_acquire (cond->__data.__g_signals + g);

12
nptl/tst-cond22.c
View File

@ -106,13 +106,13 @@ do_test (void)
status = 1;
}
printf ("cond = { 0x%x:%x, 0x%x:%x, %u/%u/%u, %u/%u/%u, %u, %u }\n",
printf ("cond = { 0x%x:%x, 0x%x:%x, %u/%u, %u/%u, %u, %u }\n",
c.__data.__wseq.__value32.__high,
c.__data.__wseq.__value32.__low,
c.__data.__g1_start.__value32.__high,
c.__data.__g1_start.__value32.__low,
c.__data.__g_signals[0], c.__data.__g_refs[0], c.__data.__g_size[0],
c.__data.__g_signals[1], c.__data.__g_refs[1], c.__data.__g_size[1],
c.__data.__g_signals[0], c.__data.__g_size[0],
c.__data.__g_signals[1], c.__data.__g_size[1],
c.__data.__g1_orig_size, c.__data.__wrefs);
if (pthread_create (&th, NULL, tf, (void *) 1l) != 0)
@ -152,13 +152,13 @@ do_test (void)
status = 1;
}
printf ("cond = { 0x%x:%x, 0x%x:%x, %u/%u/%u, %u/%u/%u, %u, %u }\n",
printf ("cond = { 0x%x:%x, 0x%x:%x, %u/%u, %u/%u, %u, %u }\n",
c.__data.__wseq.__value32.__high,
c.__data.__wseq.__value32.__low,
c.__data.__g1_start.__value32.__high,
c.__data.__g1_start.__value32.__low,
c.__data.__g_signals[0], c.__data.__g_refs[0], c.__data.__g_size[0],
c.__data.__g_signals[1], c.__data.__g_refs[1], c.__data.__g_size[1],
c.__data.__g_signals[0], c.__data.__g_size[0],
c.__data.__g_signals[1], c.__data.__g_size[1],
c.__data.__g1_orig_size, c.__data.__wrefs);
return status;

3
sysdeps/nptl/bits/thread-shared-types.h
View File

@ -95,8 +95,7 @@ struct __pthread_cond_s
{
__atomic_wide_counter __wseq;
__atomic_wide_counter __g1_start;
unsigned int __g_refs[2] __LOCK_ALIGNMENT;
unsigned int __g_size[2];
unsigned int __g_size[2] __LOCK_ALIGNMENT;
unsigned int __g1_orig_size;
unsigned int __wrefs;
unsigned int __g_signals[2];

2
sysdeps/nptl/pthread.h
View File

@ -152,7 +152,7 @@ enum
/* Conditional variable handling. */
#define PTHREAD_COND_INITIALIZER { { {0}, {0}, {0, 0}, {0, 0}, 0, 0, {0, 0} } }
#define PTHREAD_COND_INITIALIZER { { {0}, {0}, {0, 0}, 0, 0, {0, 0} } }
/* Cleanup buffers */