Revision 5b77e95dd7790ff6c8fbf1cd8d0104ebed818a03 authored by Alexander Potapenko on 02 April 2019, 11:28:13 UTC, committed by Ingo Molnar on 06 April 2019, 07:52:02 UTC
There's a number of problems with how arch/x86/include/asm/bitops.h is currently using assembly constraints for the memory region bitops are modifying: 1) Use memory clobber in bitops that touch arbitrary memory Certain bit operations that read/write bits take a base pointer and an arbitrarily large offset to address the bit relative to that base. Inline assembly constraints aren't expressive enough to tell the compiler that the assembly directive is going to touch a specific memory location of unknown size, therefore we have to use the "memory" clobber to indicate that the assembly is going to access memory locations other than those listed in the inputs/outputs. To indicate that BTR/BTS instructions don't necessarily touch the first sizeof(long) bytes of the argument, we also move the address to assembly inputs. This particular change leads to size increase of 124 kernel functions in a defconfig build. For some of them the diff is in NOP operations, other end up re-reading values from memory and may potentially slow down the execution. But without these clobbers the compiler is free to cache the contents of the bitmaps and use them as if they weren't changed by the inline assembly. 2) Use byte-sized arguments for operations touching single bytes. Passing a long value to ANDB/ORB/XORB instructions makes the compiler treat sizeof(long) bytes as being clobbered, which isn't the case. This may theoretically lead to worse code in the case of heavy optimization. Practical impact: I've built a defconfig kernel and looked through some of the functions generated by GCC 7.3.0 with and without this clobber, and didn't spot any miscompilations. However there is a (trivial) theoretical case where this code leads to miscompilation: https://lkml.org/lkml/2019/3/28/393 using just GCC 8.3.0 with -O2. It isn't hard to imagine someone writes such a function in the kernel someday. So the primary motivation is to fix an existing misuse of the asm directive, which happens to work in certain configurations now, but isn't guaranteed to work under different circumstances. [ --mingo: Added -stable tag because defconfig only builds a fraction of the kernel and the trivial testcase looks normal enough to be used in existing or in-development code. ] Signed-off-by: Alexander Potapenko <glider@google.com> Cc: <stable@vger.kernel.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: James Y Knight <jyknight@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20190402112813.193378-1-glider@google.com [ Edited the changelog, tidied up one of the defines. ] Signed-off-by: Ingo Molnar <mingo@kernel.org>
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blk-exec.c
/*
* Functions related to setting various queue properties from drivers
*/
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/blk-mq.h>
#include <linux/sched/sysctl.h>
#include "blk.h"
#include "blk-mq-sched.h"
/**
* blk_end_sync_rq - executes a completion event on a request
* @rq: request to complete
* @error: end I/O status of the request
*/
static void blk_end_sync_rq(struct request *rq, blk_status_t error)
{
struct completion *waiting = rq->end_io_data;
rq->end_io_data = NULL;
/*
* complete last, if this is a stack request the process (and thus
* the rq pointer) could be invalid right after this complete()
*/
complete(waiting);
}
/**
* blk_execute_rq_nowait - insert a request into queue for execution
* @q: queue to insert the request in
* @bd_disk: matching gendisk
* @rq: request to insert
* @at_head: insert request at head or tail of queue
* @done: I/O completion handler
*
* Description:
* Insert a fully prepared request at the back of the I/O scheduler queue
* for execution. Don't wait for completion.
*
* Note:
* This function will invoke @done directly if the queue is dead.
*/
void blk_execute_rq_nowait(struct request_queue *q, struct gendisk *bd_disk,
struct request *rq, int at_head,
rq_end_io_fn *done)
{
WARN_ON(irqs_disabled());
WARN_ON(!blk_rq_is_passthrough(rq));
rq->rq_disk = bd_disk;
rq->end_io = done;
/*
* don't check dying flag for MQ because the request won't
* be reused after dying flag is set
*/
blk_mq_sched_insert_request(rq, at_head, true, false);
}
EXPORT_SYMBOL_GPL(blk_execute_rq_nowait);
/**
* blk_execute_rq - insert a request into queue for execution
* @q: queue to insert the request in
* @bd_disk: matching gendisk
* @rq: request to insert
* @at_head: insert request at head or tail of queue
*
* Description:
* Insert a fully prepared request at the back of the I/O scheduler queue
* for execution and wait for completion.
*/
void blk_execute_rq(struct request_queue *q, struct gendisk *bd_disk,
struct request *rq, int at_head)
{
DECLARE_COMPLETION_ONSTACK(wait);
unsigned long hang_check;
rq->end_io_data = &wait;
blk_execute_rq_nowait(q, bd_disk, rq, at_head, blk_end_sync_rq);
/* Prevent hang_check timer from firing at us during very long I/O */
hang_check = sysctl_hung_task_timeout_secs;
if (hang_check)
while (!wait_for_completion_io_timeout(&wait, hang_check * (HZ/2)));
else
wait_for_completion_io(&wait);
}
EXPORT_SYMBOL(blk_execute_rq);
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