From: Heiko Carstens Date: Wed, 4 Dec 2024 11:31:00 +0000 (+0100) Subject: s390/preempt: Add comments X-Git-Tag: v6.14-rc1~200^2~37 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=a7af4fb85e208f31cc767245516460b1af287b36;p=thirdparty%2Fkernel%2Flinux.git s390/preempt: Add comments The s390 preempt_count implementation is more or less a copy of the x86 implementation using different instructions. For clarification how this works also add all comments from x86 with some minor modifications. Acked-by: Alexander Gordeev Signed-off-by: Heiko Carstens Signed-off-by: Alexander Gordeev --- diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h index 2c29bdf121272..3e3f21385db96 100644 --- a/arch/s390/include/asm/preempt.h +++ b/arch/s390/include/asm/preempt.h @@ -12,8 +12,17 @@ /* We use the MSB mostly because its available */ #define PREEMPT_NEED_RESCHED 0x80000000 + +/* + * We use the PREEMPT_NEED_RESCHED bit as an inverted NEED_RESCHED such + * that a decrement hitting 0 means we can and should reschedule. + */ #define PREEMPT_ENABLED (0 + PREEMPT_NEED_RESCHED) +/* + * We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users + * that think a non-zero value indicates we cannot preempt. + */ static __always_inline int preempt_count(void) { return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED; @@ -29,6 +38,15 @@ static __always_inline void preempt_count_set(int pc) } while (!arch_try_cmpxchg(&get_lowcore()->preempt_count, &old, new)); } +/* + * We fold the NEED_RESCHED bit into the preempt count such that + * preempt_enable() can decrement and test for needing to reschedule with a + * short instruction sequence. + * + * We invert the actual bit, so that when the decrement hits 0 we know we both + * need to resched (the bit is cleared) and can resched (no preempt count). + */ + static __always_inline void set_preempt_need_resched(void) { __atomic_and(~PREEMPT_NEED_RESCHED, &get_lowcore()->preempt_count); @@ -64,11 +82,19 @@ static __always_inline void __preempt_count_sub(int val) __preempt_count_add(-val); } +/* + * Because we keep PREEMPT_NEED_RESCHED set when we do _not_ need to reschedule + * a decrement which hits zero means we have no preempt_count and should + * reschedule. + */ static __always_inline bool __preempt_count_dec_and_test(void) { return __atomic_add(-1, &get_lowcore()->preempt_count) == 1; } +/* + * Returns true when we need to resched and can (barring IRQ state). + */ static __always_inline bool should_resched(int preempt_offset) { return unlikely(READ_ONCE(get_lowcore()->preempt_count) ==