linux-kernel - [PATCH 3.16 057/136] powerpc/opal: Fix EBUSY bug in acquiring tokens

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Message-ID: <lsq.1518323471.19123212@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:31:11 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "William A. Kennington III" <wak@...gle.com>,
        "Michael Ellerman" <mpe@...erman.id.au>
Subject: [PATCH 3.16 057/136] powerpc/opal: Fix EBUSY bug in acquiring tokens

3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "William A. Kennington III" <wak@...gle.com>

commit 71e24d7731a2903b1ae2bba2b2971c654d9c2aa6 upstream.

The current code checks the completion map to look for the first token
that is complete. In some cases, a completion can come in but the
token can still be on lease to the caller processing the completion.
If this completed but unreleased token is the first token found in the
bitmap by another tasks trying to acquire a token, then the
__test_and_set_bit call will fail since the token will still be on
lease. The acquisition will then fail with an EBUSY.

This patch reorganizes the acquisition code to look at the
opal_async_token_map for an unleased token. If the token has no lease
it must have no outstanding completions so we should never see an
EBUSY, unless we have leased out too many tokens. Since
opal_async_get_token_inrerruptible is protected by a semaphore, we
will practically never see EBUSY anymore.

Fixes: 8d7248232208 ("powerpc/powernv: Infrastructure to support OPAL async completion")
Signed-off-by: William A. Kennington III <wak@...gle.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/powerpc/platforms/powernv/opal-async.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/arch/powerpc/platforms/powernv/opal-async.c
+++ b/arch/powerpc/platforms/powernv/opal-async.c
@@ -38,18 +38,18 @@ int __opal_async_get_token(void)
 	int token;

 	spin_lock_irqsave(&opal_async_comp_lock, flags);
-	token = find_first_bit(opal_async_complete_map, opal_max_async_tokens);
+	token = find_first_zero_bit(opal_async_token_map, opal_max_async_tokens);
 	if (token >= opal_max_async_tokens) {
 		token = -EBUSY;
 		goto out;
 	}

-	if (__test_and_set_bit(token, opal_async_token_map)) {
+	if (!__test_and_clear_bit(token, opal_async_complete_map)) {
 		token = -EBUSY;
 		goto out;
 	}

-	__clear_bit(token, opal_async_complete_map);
+	__set_bit(token, opal_async_token_map);

 out:
 	spin_unlock_irqrestore(&opal_async_comp_lock, flags);