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devsd: fix possible sdbio() race with inquiry data changing (due to ahci hotplug)

the unit inquiry data might change in case the drive got pulled
with ahci. so keep track if we locked the ctl in a local stack
variable instead of relying on that the inquiry data stays the
same.

--- a/sys/src/9/port/devsd.c

+++ b/sys/src/9/port/devsd.c

@@ -770,7 +770,7 @@

 static long

 sdbio(Chan* c, int write, char* a, long len, uvlong off)

 {

-	int nchange, hard, allocd;

+	int nchange, hard, allocd, locked;

 	long l;

 	uchar *b;

 	SDpart *pp;

@@ -832,7 +832,8 @@

 		poperror();

 		return 0;

 	}

-	if(!(unit->inquiry[1] & 0x80)){

+	locked = (unit->inquiry[1] & 0x80) != 0;

+	if(!locked){

 		qunlock(&unit->ctl);

 		poperror();

 	}

@@ -854,7 +855,7 @@

 	if(waserror()){

 		if(allocd)

 			sdfree(b);

-		if(!(unit->inquiry[1] & 0x80))

+		if(!locked)

 			decref(&sdev->r);		/* gadverdamme! */

 		nexterror();

 	}

@@ -896,7 +897,7 @@

 		sdfree(b);

 	poperror();

-	if(unit->inquiry[1] & 0x80){

+	if(locked){

 		qunlock(&unit->ctl);

 		poperror();

 	}

-- 

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