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Subject[PATCH for 4.18 3/6] rseq: uapi: update uapi comments
Date
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@efficios.com>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Paul Turner <pjt@google.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Andi Kleen <andi@firstfloor.org>
CC: Dave Watson <davejwatson@fb.com>
CC: Chris Lameter <cl@linux.com>
CC: Ingo Molnar <mingo@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Ben Maurer <bmaurer@fb.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Josh Triplett <josh@joshtriplett.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Russell King <linux@arm.linux.org.uk>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Will Deacon <will.deacon@arm.com>
CC: Michael Kerrisk <mtk.manpages@gmail.com>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: linux-api@vger.kernel.org
---
include/uapi/linux/rseq.h | 69 ++++++++++++++++++++++++-----------------------
kernel/rseq.c | 2 +-
2 files changed, 37 insertions(+), 34 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))));
diff --git a/kernel/rseq.c b/kernel/rseq.c
index 2c8463acb50d..2a7748675be7 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -201,7 +201,7 @@ static int clear_rseq_cs(struct task_struct *t)
* of code outside of the rseq assembly block. This performs
* a lazy clear of the rseq_cs field.
*
- * Set rseq_cs to NULL with single-copy atomicity.
+ * Set rseq_cs to NULL.
*/
return put_user(0UL, &t->rseq->rseq_cs);
}
--
2.11.0
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