diff options
author | Soby Mathew <soby.mathew@arm.com> | 2019-09-25 14:03:41 +0100 |
---|---|---|
committer | Olivier Deprez <olivier.deprez@arm.com> | 2019-10-04 10:19:35 +0200 |
commit | c97cba4ea44910df1f7b1af5dba79013fb44c383 (patch) | |
tree | 9d2965a2b9610413dea548a95e2f5094fc84b6e8 /lib/locks | |
parent | ace23683beb81354d6edbc61c087ab8c384d0631 (diff) |
Fix the CAS spinlock implementation
Make the spinlock implementation use ARMv8.1-LSE CAS instruction based
on a platform build option. The CAS-based implementation used to be
unconditionally selected for all ARM8.1+ platforms.
The previous CAS spinlock implementation had a bug wherein the spin_unlock()
implementation had an `sev` after `stlr` which is not sufficient. A dsb is
needed to ensure that the stlr completes prior to the sev. Having a dsb is
heavyweight and a better solution would be to use load exclusive semantics
to monitor the lock and wake up from wfe when a store happens to the lock.
The patch implements the same.
Change-Id: I5283ce4a889376e4cc01d1b9d09afa8229a2e522
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Diffstat (limited to 'lib/locks')
-rw-r--r-- | lib/locks/exclusive/aarch64/spinlock.S | 53 |
1 files changed, 18 insertions, 35 deletions
diff --git a/lib/locks/exclusive/aarch64/spinlock.S b/lib/locks/exclusive/aarch64/spinlock.S index d0569f1c..e941b8a3 100644 --- a/lib/locks/exclusive/aarch64/spinlock.S +++ b/lib/locks/exclusive/aarch64/spinlock.S @@ -9,56 +9,38 @@ .globl spin_lock .globl spin_unlock -#if ARM_ARCH_AT_LEAST(8, 1) +#if USE_SPINLOCK_CAS +#if !ARM_ARCH_AT_LEAST(8, 1) +#error USE_SPINLOCK_CAS option requires at least an ARMv8.1 platform +#endif /* * When compiled for ARMv8.1 or later, choose spin locks based on Compare and * Swap instruction. */ -# define USE_CAS 1 - -/* - * Lock contenders using CAS, upon failing to acquire the lock, wait with the - * monitor in open state. Therefore, a normal store upon unlocking won't - * generate an SEV. Use explicit SEV instruction with CAS unlock. - */ -# define COND_SEV() sev - -#else - -# define USE_CAS 0 - -/* - * Lock contenders using exclusive pairs, upon failing to acquire the lock, wait - * with the monitor in exclusive state. A normal store upon unlocking will - * implicitly generate an envent; so, no explicit SEV with unlock is required. - */ -# define COND_SEV() - -#endif - -#if USE_CAS /* * Acquire lock using Compare and Swap instruction. * - * Compare for 0 with acquire semantics, and swap 1. Wait until CAS returns - * 0. + * Compare for 0 with acquire semantics, and swap 1. If failed to acquire, use + * load exclusive semantics to monitor the address and enter WFE. * * void spin_lock(spinlock_t *lock); */ func spin_lock mov w2, #1 - sevl -1: +1: mov w1, wzr +2: casa w1, w2, [x0] + cbz w1, 3f + ldxr w1, [x0] + cbz w1, 2b wfe - mov w1, wzr - casa w1, w2, [x0] - cbnz w1, 1b + b 1b +3: ret endfunc spin_lock -#else /* !USE_CAS */ +#else /* !USE_SPINLOCK_CAS */ /* * Acquire lock using load-/store-exclusive instruction pair. @@ -76,17 +58,18 @@ l2: ldaxr w1, [x0] ret endfunc spin_lock -#endif /* USE_CAS */ +#endif /* USE_SPINLOCK_CAS */ /* * Release lock previously acquired by spin_lock. * - * Unconditionally write 0, and conditionally generate an event. + * Use store-release to unconditionally clear the spinlock variable. + * Store operation generates an event to all cores waiting in WFE + * when address is monitored by the global monitor. * * void spin_unlock(spinlock_t *lock); */ func spin_unlock stlr wzr, [x0] - COND_SEV() ret endfunc spin_unlock |