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Subject[PATCH] [2.5.36] USER_HZ & SG_[GS]ET_TIMEOUT
Hi!

One of the places in the kernel where HZ is interfaced with userspace
is in the SG driver. With HZ different from USER_HZ this may result
in all kinds of unexpected timeouts when using SG.

This patch is an attempt to fix this by transforming USER_HZ based timing to
HZ based timing before storing it in timeout. To make sure that SG_GET_TIMEOUT
and SG_SET_TIMEOUT behave consistently a field timeout_user is added which
stores the exact value that's passed by SG_SET_TIMEOUT and it's returned on
SG_GET_TIMEOUT.

Rolf

--- linux-2.5.36.orig/drivers/scsi/sg.c Sun Sep 8 09:00:32 2002
+++ linux-2.5.36/drivers/scsi/sg.c Wed Sep 18 19:47:15 2002
@@ -82,6 +82,19 @@

#define SG_MAX_DEVS_MASK ((1U << KDEV_MINOR_BITS) - 1)

+/*
+ * Suppose you want to calculate the formula muldiv(x,m,d)=int(x * m / d)
+ * Then when using 32 bit integers x * m may overflow during the calculation.
+ * Replacing muldiv(x) by muldiv(x)=((x % d) * m) / d + int(x / d) * m
+ * calculates the same, but prevents the overflow when both m and d
+ * are "small" numbers (like HZ and USER_HZ).
+ * Of course an overflow is inavoidable if the result of muldiv doesn't fit
+ * in 32 bits.
+ */
+#define MULDIV(X,MUL,DIV) ((((X % DIV) * MUL) / DIV) + ((X / DIV) * MUL))
+
+#define SG_DEFAULT_TIMEOUT MULDIV(SG_DEFAULT_TIMEOUT_USER, HZ, USER_HZ)
+
int sg_big_buff = SG_DEF_RESERVED_SIZE;
/* N.B. This variable is readable and writeable via
/proc/scsi/sg/def_reserved_size . Each time sg_open() is called a buffer
@@ -150,7 +163,8 @@
struct sg_device *parentdp; /* owning device */
wait_queue_head_t read_wait; /* queue read until command done */
rwlock_t rq_list_lock; /* protect access to list in req_arr */
- int timeout; /* defaults to SG_DEFAULT_TIMEOUT */
+ int timeout; /* defaults to SG_DEFAULT_TIMEOUT */
+ int timeout_user; /* defaults to SG_DEFAULT_TIMEOUT_USER */
Sg_scatter_hold reserve; /* buffer held for this file descriptor */
unsigned save_scat_len; /* original length of trunc. scat. element */
Sg_request *headrp; /* head of request slist, NULL->empty */
@@ -790,10 +804,15 @@
return result;
if (val < 0)
return -EIO;
- sfp->timeout = val;
+ if (val >= MULDIV (INT_MAX, USER_HZ, HZ))
+ val = MULDIV (INT_MAX, USER_HZ, HZ);
+ sfp->timeout_user = val;
+ sfp->timeout = MULDIV (val, HZ, USER_HZ);
+
return 0;
case SG_GET_TIMEOUT: /* N.B. User receives timeout as return value */
- return sfp->timeout; /* strange ..., for backward compatibility */
+ /* strange ..., for backward compatibility */
+ return sfp->timeout_user;
case SG_SET_FORCE_LOW_DMA:
result = get_user(val, (int *) arg);
if (result)
@@ -2431,6 +2450,7 @@
sfp->rq_list_lock = RW_LOCK_UNLOCKED;

sfp->timeout = SG_DEFAULT_TIMEOUT;
+ sfp->timeout_user = SG_DEFAULT_TIMEOUT_USER;
sfp->force_packid = SG_DEF_FORCE_PACK_ID;
sfp->low_dma = (SG_DEF_FORCE_LOW_DMA == 0) ?
sdp->device->host->unchecked_isa_dma : 1;
--- linux-2.5.36.orig/include/scsi/sg.h Sun Sep 8 09:00:19 2002
+++ linux-2.5.36/include/scsi/sg.h Wed Sep 18 10:27:12 2002
@@ -304,7 +304,12 @@


/* Defaults, commented if they differ from original sg driver */
-#define SG_DEFAULT_TIMEOUT (60*HZ) /* HZ == 'jiffies in 1 second' */
+#ifdef __KERNEL__
+#define SG_DEFAULT_TIMEOUT_USER (60*USER_HZ) /* HZ == 'jiffies in 1 second' */
+#else
+#define SG_DEFAULT_TIMEOUT (60*HZ) /* HZ == 'jiffies in 1 second' */
+#endif
+
#define SG_DEF_COMMAND_Q 0 /* command queuing is always on when
the new interface is used */
#define SG_DEF_UNDERRUN_FLAG 0
-
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