Update rseq uapi header comments to reflect that user-space need to do thread-local loads/stores from/to the struct rseq fields. As a consequence of this added requirement, the kernel does not need to perform loads/stores with single-copy atomicity. Update the comment associated to the "flags" fields to describe more accurately that it's only useful to facilitate single-stepping through rseq critical sections with debuggers. Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx> CC: Paul Turner <pjt@xxxxxxxxxx> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx> CC: Andi Kleen <andi@xxxxxxxxxxxxxx> CC: Dave Watson <davejwatson@xxxxxx> CC: Chris Lameter <cl@xxxxxxxxx> CC: Ingo Molnar <mingo@xxxxxxxxxx> CC: "H. Peter Anvin" <hpa@xxxxxxxxx> CC: Ben Maurer <bmaurer@xxxxxx> CC: Steven Rostedt <rostedt@xxxxxxxxxxx> CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> CC: Russell King <linux@xxxxxxxxxxxxxxxx> CC: Catalin Marinas <catalin.marinas@xxxxxxx> CC: Will Deacon <will.deacon@xxxxxxx> CC: Michael Kerrisk <mtk.manpages@xxxxxxxxx> CC: Boqun Feng <boqun.feng@xxxxxxxxx> CC: linux-api@xxxxxxxxxxxxxxx --- include/uapi/linux/rseq.h | 69 ++++++++++++++++++++++++----------------------- 1 file changed, 36 insertions(+), 33 deletions(-) diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h index 519ad6e176d1..bf4188c13bec 100644 --- a/include/uapi/linux/rseq.h +++ b/include/uapi/linux/rseq.h @@ -67,28 +67,30 @@ struct rseq_cs { struct rseq { /* * Restartable sequences cpu_id_start field. Updated by the - * kernel, and read by user-space with single-copy atomicity - * semantics. Aligned on 32-bit. Always contains a value in the - * range of possible CPUs, although the value may not be the - * actual current CPU (e.g. if rseq is not initialized). This - * CPU number value should always be compared against the value - * of the cpu_id field before performing a rseq commit or - * returning a value read from a data structure indexed using - * the cpu_id_start value. + * kernel. Read by user-space with single-copy atomicity + * semantics. This field should only be read by the thread which + * registered this data structure. Aligned on 32-bit. Always + * contains a value in the range of possible CPUs, although the + * value may not be the actual current CPU (e.g. if rseq is not + * initialized). This CPU number value should always be compared + * against the value of the cpu_id field before performing a rseq + * commit or returning a value read from a data structure indexed + * using the cpu_id_start value. */ __u32 cpu_id_start; /* - * Restartable sequences cpu_id field. Updated by the kernel, - * and read by user-space with single-copy atomicity semantics. - * Aligned on 32-bit. Values RSEQ_CPU_ID_UNINITIALIZED and - * RSEQ_CPU_ID_REGISTRATION_FAILED have a special semantic: the - * former means "rseq uninitialized", and latter means "rseq - * initialization failed". This value is meant to be read within - * rseq critical sections and compared with the cpu_id_start - * value previously read, before performing the commit instruction, - * or read and compared with the cpu_id_start value before returning - * a value loaded from a data structure indexed using the - * cpu_id_start value. + * Restartable sequences cpu_id field. Updated by the kernel. + * Read by user-space with single-copy atomicity semantics. This + * field should only be read by the thread which registered this + * data structure. Aligned on 32-bit. Values + * RSEQ_CPU_ID_UNINITIALIZED and RSEQ_CPU_ID_REGISTRATION_FAILED + * have a special semantic: the former means "rseq uninitialized", + * and latter means "rseq initialization failed". This value is + * meant to be read within rseq critical sections and compared + * with the cpu_id_start value previously read, before performing + * the commit instruction, or read and compared with the + * cpu_id_start value before returning a value loaded from a data + * structure indexed using the cpu_id_start value. */ __u32 cpu_id; /* @@ -105,27 +107,28 @@ struct rseq { * targeted by the rseq_cs. Also needs to be set to NULL by user-space * before reclaiming memory that contains the targeted struct rseq_cs. * - * Read and set by the kernel with single-copy atomicity semantics. - * Set by user-space with single-copy atomicity semantics. Aligned - * on 64-bit. + * Read and set by the kernel. Set by user-space with single-copy + * atomicity semantics. This field should only be updated by the + * thread which registered this data structure. Aligned on 64-bit. */ LINUX_FIELD_u32_u64(rseq_cs); /* - * - RSEQ_DISABLE flag: + * Restartable sequences flags field. + * + * This field should only be updated by the thread which + * registered this data structure. Read by the kernel. + * Mainly used for single-stepping through rseq critical sections + * with debuggers. * - * Fallback fast-track flag for single-stepping. - * Set by user-space if lack of progress is detected. - * Cleared by user-space after rseq finish. - * Read by the kernel. * - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT - * Inhibit instruction sequence block restart and event - * counter increment on preemption for this thread. + * Inhibit instruction sequence block restart on preemption + * for this thread. * - RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL - * Inhibit instruction sequence block restart and event - * counter increment on signal delivery for this thread. + * Inhibit instruction sequence block restart on signal + * delivery for this thread. * - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE - * Inhibit instruction sequence block restart and event - * counter increment on migration for this thread. + * Inhibit instruction sequence block restart on migration for + * this thread. */ __u32 flags; } __attribute__((aligned(4 * sizeof(__u64)))); -- 2.11.0 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html