'\" t
.\"     Title: struct fence
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: Device drivers infrastructure
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "STRUCT FENCE" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "Device drivers infrastructure"
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.SH "NAME"
struct_fence \- software synchronization primitive
.SH "SYNOPSIS"
.sp
.nf
struct fence {
  struct kref refcount;
  const struct fence_ops * ops;
  struct rcu_head rcu;
  struct list_head cb_list;
  spinlock_t * lock;
  u64 context;
  unsigned seqno;
  unsigned long flags;
  ktime_t timestamp;
  int status;
};  
.fi
.SH "MEMBERS"
.PP
refcount
.RS 4
refcount for this fence
.RE
.PP
ops
.RS 4
fence_ops associated with this fence
.RE
.PP
rcu
.RS 4
used for releasing fence with kfree_rcu
.RE
.PP
cb_list
.RS 4
list of all callbacks to call
.RE
.PP
lock
.RS 4
spin_lock_irqsave used for locking
.RE
.PP
context
.RS 4
execution context this fence belongs to, returned by
\fBfence_context_alloc\fR
.RE
.PP
seqno
.RS 4
the sequence number of this fence inside the execution context, can be compared to decide which fence would be signaled later\&.
.RE
.PP
flags
.RS 4
A mask of FENCE_FLAG_* defined below
.RE
.PP
timestamp
.RS 4
Timestamp when the fence was signaled\&.
.RE
.PP
status
.RS 4
Optional, only valid if < 0, must be set before calling fence_signal, indicates that the fence has completed with an error\&.
.RE
.SH "DESCRIPTION"
.PP
the flags member must be manipulated and read using the appropriate atomic ops (bit_*), so taking the spinlock will not be needed most of the time\&.
.PP
FENCE_FLAG_SIGNALED_BIT \- fence is already signaled FENCE_FLAG_ENABLE_SIGNAL_BIT \- enable_signaling might have been called* FENCE_FLAG_USER_BITS \- start of the unused bits, can be used by the implementer of the fence for its own purposes\&. Can be used in different ways by different fence implementers, so do not rely on this\&.
.PP
*) Since atomic bitops are used, this is not guaranteed to be the case\&. Particularly, if the bit was set, but fence_signal was called right before this bit was set, it would have been able to set the FENCE_FLAG_SIGNALED_BIT, before enable_signaling was called\&. Adding a check for FENCE_FLAG_SIGNALED_BIT after setting FENCE_FLAG_ENABLE_SIGNAL_BIT closes this race, and makes sure that after fence_signal was called, any enable_signaling call will have either been completed, or never called at all\&.
.SH "COPYRIGHT"
.br