[Devel] Re: [PATCH] c/r: replace error_sem with an event completion

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> (This patch applies on top of ckpt-v20-rc1)
> 
> The value of ctx->errno is protected by a semaphore to ensure that
> processes do not pique at it prematurely (that is - after the error
> bit is set, but before the value is committed).
> 
> But, lockdep sometimes complains when an error is detected from a
> legitimately failed restart attempt. This is because we release
> ctx->errno_sem in a task context different from the one in which it
> was acquired.
> 
> Replace the semaphore with an event completion: this sort of
> synchronization is what completion constructs are all about.
> 
> Originally reported by Nathan Lynch.
> 
> Signed-off-by: Oren Laadan <orenl at cs.columbia.edu>

Acked-by: Serge Hallyn <serue at us.ibm.com>

> ---
>  checkpoint/sys.c                 |   13 ++++---------
>  include/linux/checkpoint.h       |    5 ++---
>  include/linux/checkpoint_types.h |    2 +-
>  3 files changed, 7 insertions(+), 13 deletions(-)
> 
> diff --git a/checkpoint/sys.c b/checkpoint/sys.c
> index b605784..fe85ca7 100644
> --- a/checkpoint/sys.c
> +++ b/checkpoint/sys.c
> @@ -280,8 +280,7 @@ static struct ckpt_ctx *ckpt_ctx_alloc(int fd, unsigned long uflags,
>  	init_waitqueue_head(&ctx->ghostq);
>  	init_completion(&ctx->complete);
> 
> -	init_rwsem(&ctx->errno_sem);
> -	down_write(&ctx->errno_sem);
> +	init_completion(&ctx->errno_sync);
> 
>  #ifdef CONFIG_CHECKPOINT_DEBUG
>  	INIT_LIST_HEAD(&ctx->task_status);
> @@ -343,11 +342,8 @@ void ckpt_set_error(struct ckpt_ctx *ctx, int err)
>  	/* atomically set ctx->errno */
>  	if (!ckpt_test_and_set_ctx_kflag(ctx, CKPT_CTX_ERROR)) {
>  		ctx->errno = err;
> -		/*
> -		 * We initialized ctx->errno_sem write-held to prevent
> -		 * other tasks from reading ctx->errno prematurely.
> -		 */
> -		up_write(&ctx->errno_sem);
> +		/* make ctx->errno visible to all other tasks */
> +		complete_all(&ctx->errno_sync);
>  		/* on restart, notify all tasks in restarting subtree */
>  		if (ctx->kflags & CKPT_CTX_RESTART)
>  			restore_notify_error(ctx);
> @@ -357,8 +353,7 @@ void ckpt_set_error(struct ckpt_ctx *ctx, int err)
>  void ckpt_set_success(struct ckpt_ctx *ctx)
>  {
>  	ckpt_set_ctx_kflag(ctx, CKPT_CTX_SUCCESS);
> -	/* avoid warning "lock still held" when freeing (was write-held) */
> -	up_write(&ctx->errno_sem);
> +	complete_all(&ctx->errno_sync);
>  }
> 
>  /* helpers to handler log/dbg/err messages */
> diff --git a/include/linux/checkpoint.h b/include/linux/checkpoint.h
> index a25bac1..a8b9e21 100644
> --- a/include/linux/checkpoint.h
> +++ b/include/linux/checkpoint.h
> @@ -160,10 +160,9 @@ static inline int ckpt_get_error(struct ckpt_ctx *ctx)
>  {
>  	/*
>  	 * We may notice CKPT_CTX_ERROR before ctx->errno is set, but
> -	 * ctx->errno_sem remains (write) held until after it's done.
> +	 * ctx->errno_sync remains not-completed until after it's done.
>  	 */
> -	if (!ctx->errno)
> -		down_read(&ctx->errno_sem);
> +	wait_for_completion(&ctx->errno_sync);
>  	return ctx->errno;
>  }
> 
> diff --git a/include/linux/checkpoint_types.h b/include/linux/checkpoint_types.h
> index efc9357..e9cc1d8 100644
> --- a/include/linux/checkpoint_types.h
> +++ b/include/linux/checkpoint_types.h
> @@ -61,7 +61,7 @@ struct ckpt_ctx {
>  	char err_string[256];	/* checkpoint: error string */
> 
>  	int errno;		/* errno that caused failure */
> -	struct rw_semaphore errno_sem;	/* protect errno setting */
> +	struct completion errno_sync;	/* protect errno setting */
> 
>  	struct list_head pgarr_list;	/* page array to dump VMA contents */
>  	struct list_head pgarr_pool;	/* pool of empty page arrays chain */
> -- 
> 1.6.3.3
> 
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