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rseq: uapi: Update uapi comments 

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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-api@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Watson <davejwatson@fb.com>
Cc: Paul Turner <pjt@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Chris Lameter <cl@linux.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: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20180709195155.7654-4-mathieu.desnoyers@efficios.com
This commit is contained in:
Mathieu Desnoyers 2018-07-09 15:51:52 -04:00 committed by Thomas Gleixner
parent 8f28177014
commit 0fb9a1abc8
2 changed files with 37 additions and 34 deletions
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69
include/uapi/linux/rseq.h
View File

@ -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
kernel/rseq.c
View File

@ -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);
}