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locks: fix two potential overflows in the lock delay code

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With large numbers of CPUs, the calculation of the maximum lock delay
could overflow, leading to an unexpectedly low delay. In fact, the
maximum delay would calculate to 0 on systems with between 128 and
255 cores (inclusive). Also, when calculating the new delay in
lock_delay(), the delay would overflow if the old delay was >= 32,768.

This commit fixes these two overflows. It also updates the maximum
delay from 32,678 to SHRT_MAX.

Reviewed by:	gallatin, jhb, mjg
Fixes:	6b8dd26e7c ("locks: convert delay times to u_short")
MFC after:	2 weeks
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D39372
This commit is contained in:
Jonathan T. Looney 2023-05-09 16:20:49 +00:00
parent bf864c3ed5
commit 8a16fb4730
2 changed files with 4 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
sys/kern/subr_lock.c
View File

@ -121,14 +121,14 @@ void
lock_delay(struct lock_delay_arg *la)
{
struct lock_delay_config *lc = la->config;
u_short i;
u_int i;
for (i = la->delay; i > 0; i--)
cpu_spinwait();
la->spin_cnt += la->delay;
la->delay <<= 1;
if (__predict_false(la->delay > lc->max))
if (__predict_false(la->delay > (u_int)lc->max))
la->delay = lc->max;
}
@ -148,9 +148,7 @@ lock_delay_default_init(struct lock_delay_config *lc)
{
lc->base = 1;
lc->max = lock_roundup_2(mp_ncpus) * 256;
if (lc->max > 32678)
lc->max = 32678;
lc->max = min(lock_roundup_2(mp_ncpus) * 256, SHRT_MAX);
}
struct lock_delay_config __read_frequently locks_delay;

2
sys/sys/lock.h
View File

@ -182,7 +182,7 @@ extern u_short locks_delay_loops;
struct lock_delay_arg {
struct lock_delay_config *config;
u_short delay;
u_int delay;
u_int spin_cnt;
};