Author: kevans Date: Wed Oct 2 15:13:40 2019 New Revision: 353008 URL: https://svnweb.freebsd.org/changeset/base/353008
Log: mips: fcmpset: do not spin on sc failure For ll/sc architectures, atomic(9) allows failure modes where *old == val due to write failure and callers should compensate for this. Do not retry on failure, just leave 0 in ret and fail the operation if we couldn't sc it. This lets the caller determine if it should retry or not. Reviewed by: kib Looks ok: imp Differential Revision: https://reviews.freebsd.org/D21836 Modified: head/sys/mips/include/atomic.h Modified: head/sys/mips/include/atomic.h ============================================================================== --- head/sys/mips/include/atomic.h Wed Oct 2 13:46:40 2019 (r353007) +++ head/sys/mips/include/atomic.h Wed Oct 2 15:13:40 2019 (r353008) @@ -397,18 +397,25 @@ atomic_fcmpset_32(__volatile uint32_t *p, uint32_t *cm { int ret; + /* + * The following sequence (similar to that in atomic_fcmpset_64) will + * attempt to update the value of *p with newval if the comparison + * succeeds. Note that they'll exit regardless of whether the store + * actually succeeded, leaving *cmpval untouched. This is in line with + * the documentation of atomic_fcmpset_<type>() in atomic(9) for ll/sc + * architectures. + */ __asm __volatile ( - "1:\n\t" "ll %0, %1\n\t" /* load old value */ - "bne %0, %4, 2f\n\t" /* compare */ + "bne %0, %4, 1f\n\t" /* compare */ "move %0, %3\n\t" /* value to store */ "sc %0, %1\n\t" /* attempt to store */ - "beqz %0, 1b\n\t" /* if it failed, spin */ - "j 3f\n\t" - "2:\n\t" + "j 2f\n\t" /* exit regardless of success */ + "nop\n\t" /* avoid delay slot accident */ + "1:\n\t" "sw %0, %2\n\t" /* save old value */ "li %0, 0\n\t" - "3:\n" + "2:\n" : "=&r" (ret), "+m" (*p), "=m" (*cmpval) : "r" (newval), "r" (*cmpval) : "memory"); @@ -508,17 +515,16 @@ atomic_fcmpset_64(__volatile uint64_t *p, uint64_t *cm int ret; __asm __volatile ( - "1:\n\t" "lld %0, %1\n\t" /* load old value */ - "bne %0, %4, 2f\n\t" /* compare */ + "bne %0, %4, 1f\n\t" /* compare */ "move %0, %3\n\t" /* value to store */ "scd %0, %1\n\t" /* attempt to store */ - "beqz %0, 1b\n\t" /* if it failed, spin */ - "j 3f\n\t" - "2:\n\t" + "j 2f\n\t" /* exit regardless of success */ + "nop\n\t" /* avoid delay slot accident */ + "1:\n\t" "sd %0, %2\n\t" /* save old value */ "li %0, 0\n\t" - "3:\n" + "2:\n" : "=&r" (ret), "+m" (*p), "=m" (*cmpval) : "r" (newval), "r" (*cmpval) : "memory"); _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
